Grifter, how many BlackBerry users are you going to deploy? MSDE 2000, although is free, but there are many limitations which you should be aware of:
1. MSDE is limited to a maximum database size of 2 GB. It's important to note that the 2-GB limitation is per database not per instance.
2. There is work governor that throttles performance by stalling a user connection for a few milliseconds each time the connection requests a read or write operation on a database when there are more than 8 concurrent connections. Sharing BES with an existing application already using MSDE will contest for connections and may affected your BES performance.
3. Memory at least 2 GB. Multiple instance may or may not increase the memory usage, again may cause performance issue. Do not forget - Domino server is another memory pig.
You may like to consider upgrading your current MSDE 2000 to SQL Server 2005 Express. There are many improvements and advantages.
See
"Upgrading MSDE 2000 to SQL Server 2005 Express" for more information.